1. Field of the Invention
The present invention generally relates to methods for preparing quaternary ammonium salts and low hazard corrosion inhibitors using those salts. More specifically, the present invention is directed to methods particularly useful for producing pyridinium and quinolinium salts, to low hazard corrosion inhibitors using those salts and to aqueous cleaning solutions using those inhibitors.
2. Description of the Background
Scale comprised of insoluble salts is typically found on the surface of all types of metal equipment in which water is evaporated or heat transfer occurs. These salt deposits are particularly undesirable because of their negative impact on the heat transfer efficiency of the equipment. Because the equipment loses heat transfer efficiency as these deposits build up, it is necessary to periodically clean the equipment to remove the deposits. Industrial cleaning service companies often provide the required cleaning services for this equipment, e.g., the boilers and heat transfer equipment of utilities and industrial plants.
The undesirable scales which must be removed generally comprise calcium and magnesium salts deposited during the evaporation of hard water. Exemplary of these scales are deposits including calcium carbonate, calcium sulfate, calcium phosphate and calcium oxylate. While calcium and magnesium salts comprise the majority of these deposits, salts of other materials may be encountered. Scales high in iron content, e.g., scales including magnetite or hematite, often must be cleaned.
The conventional cleaning operations rely upon the circulation of aqueous cleaning solutions through the equipment, e.g., boilers, heat exchangers and associated piping in an effort to dissolve the salt deposits comprising the scale. Often, these cleaning solutions are heated to temperatures above the boiling point of water. In many early cleaning efforts highly acidic solutions were circulated through the equipment to dissolve the calcium and magnesium salts found in the hard water scale and the magnetite and hematite deposits encountered in high iron scales. As cleaning operations became more sophisticated, solutions containing agents capable of complexing the metals associated with the deposited salts were circulated in order to loosen and dissolve the scale. Ammonia has been used as an alkaline complexing agent for this purpose. See, e.g., the disclosure in U.S. Pat. No. 3,413,160. More recent developments have included complexing agents based upon ethylenediaminetetraacetic acid and related compounds.
Because many of these cleaning solutions are, themselves, corrosive to the metal components of the equipment being cleaned, the solutions must include appropriate corrosion inhibitors. For example, aliphatic pyridinium and quinolinium salts, together with sulphur-containing compounds, have been employed successfully as corrosion inhibitors in these solutions. See, e.g., the disclosure in U.S. Pat. No. 4,637,899 which is incorporated herein by reference. While these corrosion inhibitors provide the desired protection of the metal surfaces, they are often contaminated with unsafe and/or toxic byproducts, e.g., solvents and unreacted reactants, resulting from the methods by which they were prepared. Accordingly, these corrosion inhibitors and cleaning solutions produced therefrom can present dangers to the employees working with them. Another danger associated with the use of these corrosion inhibitors is the low flash point, often less than 100.degree. F., resulting from the solvents and resistants which were used in their manufacture. Still another danger may result from the toxicity of its sulfur-containing compounds employed in these corrosion inhibitors. For example, ethylene glycol monobutyl ether is a toxic chemical used as a solvent and carried along with the aliphatic pyridinium salts used in the methods and corrosion inhibitors disclosed in the '899 patent. Thiourea is an undesirable sulfur-containing compound typically used in these inhibitors and methods.
As environmental and worker safety concerns have increased, the need to employ less toxic corrosion inhibitors and cleaning solutions has increased. Further, as OSHA requirements and worker safety issues have evolved, the benefits of employing corrosion inhibitors and cleaning solutions with higher flash points has become clear. The industrial cleaning service industry has continued to seek improved corrosion inhibitors and cleaning solutions for use in commercial, scale cleaning operations. The known methods and solutions have not solved these problems. Thus, there has been a long felt but unfulfilled need in the industrial cleaning service industry for less toxic and safer corrosion inhibitors and cleaning solutions. The present invention solves those needs.